The present invention relates to a method and apparatus for detecting and regulating a focused condition of an electron beam emitted from an electron gun of an electron beam welding apparatus and directed onto a workpiece, and, more particularly, to a method and apparatus for detecting and regulating a focus point on the workpiece prior to commencing a welding operation.
It has been usual that the focusing of an electron beam is detected and regulated by directing an electron beam onto a dummy block of a material such as tungsten or copper and regulating the current flowing through an electromagnetic lens coil while observing the spot of the electron beam or a melt pond on the surface of the workpiece or the dummy block. Such a conventional method is disadvantageous in that the detection and regulation of the electron beam depend upon the operator's eyes.
Particularly, in an actual welding operation, the focus position of the electron beam on the workpiece affects the profile of the weld portion. FIGS. 1A through 3A illustrate the focus position of electron beams and FIGS. 1B through 3B corresponding profiles of weld portions.
In FIGS. 1A and 1B, when the electron beam EB is focused exactly on the surface of the worpiece WK, the welding is made to a certain depth with a small diameter. On the other hand, when the focus point is inside the workpiece WK, as shown in FIGS. 2A and 2B, the welding depth becomes much smaller than in the case shown in FIGS. 1A and 1B, but with a larger diameter. If the focus point is above the workpiece surface, as shown in FIGS. 3A and 3B, the weld profile becomes similar to a shape of a short and thick nail. The weld profiles shown in FIGS. 2B and 3B are not acceptable.
In order to resolve this problem, Japanese Laid-Open Patent Application No. 48-339 proposes a technique according to which light produced by an electron beam and reflected from the portion of a surface of a workpiece onto which the electron beam is directed is sensed by an indicator and focusing regulation is performed accordingly. This technique is more effective than visual observance of the melt pond. However, the energy of the electron beam used for regulation is much smaller than that of the electron beam used to weld the workpiece, and thus it is necessary to increase the electron beam energy after regulation has been completed. Moreover, possible variation of the focusing position due to an increase of the electron beam energy is not taken into account. Further, although the precision of the focusing position regulation may be higher than that obtainable by visual observance, the technique is still disadvantageous since the precision depends on the skill of the operator. Eggers et al. propose in their article "Automatic Focusing in High Power Electron Beam Welding", IIW-Doc. No. IV/57/71, April 1971, a technique in which electrons emitted from a workpiece irradiated with an electron beam is utilized to detect the focus position while the focus position is changed. A pair of electron collectors are used to detect the focus position. Since this technique uses the welding beam directly, it is necessary to use complicated electronics circuits in order to avoid undesirable welding conditions.